Problem with Vertex Displacement Mapping (GLSL)

I can load images of colormap and displacementmap into glTexImage2D() without any problem. (displacement texture filter is nearest already)

The problem is I did not see any bump/displace on my plane.... I could see it be mapped with correct colorMap.
I felt that the whole plane is move along z-axis (normal) according to vertex shader when I changed value that multiply df at line "newVertexPos = vec4(gl_Normal * df * 100.0, 0.0) + gl_Vertex;"

There is nothing wrong that's what you asked to code to do - all your vertices have the same normal which point along the z-axis. In you shader you multiply this normal by a values and add it to
the vertex so you will get a displacement along the z axis. Try putting different values in each normal like (0,1,0), (0.5,0.5,0) , (0,0.5,0.5), (0,0,0) and see what happens